Method and system for television communication



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Patented Jan. 9, 1940 UNITED STATES 5 METHOD AND SYSTEM FOR TELEVISIONCODIMUNICATION Allen B. Du Mont, Upper Montclair, N. 1., assignor toAllen B. Du Mont Laboratories, Inc., Passalc, N. 1., a corporation ofDelaware Application January 26, 1938, Serial No. 188,981

- 4 Claims.

My invention relates to improvements in methods and systems fortelevision communication.

In the more successful methods and systems proposed heretofore fortelevision communication, and employing a cathode-ray pick-up tube atthe transmitter and a cathode-ray viewing tube at the receiver, it hasbeen required .that there be at each station two circuits for deflectingthe scanning ray at the line and field fre-' quencies, respectively.Also, it has been required that line and field pulses be transmitted onthe same carrier wave as the video signals, and these pulses have beenused at the receiver for synchronization. In these prior methods andsystems, not only has difficulty been encountered in .holding thedeflecting circuits at the receiver locked in step with those at thetransmitter, but a substantial portion of the transmitted energy hasbeen expended in transmitting the synchronizing pulses.

With the foregoing in mind, it is one of the objects of my invention toprovide an improved method and system for television communicationemploying cathode-ray tubes at both stations, and in which nosynchronizing pulses are required to be transmitted to the receiver,-andin which either the deflection circuit at the line frequency or bothdeflection circuits at the linewaves at therespective line and fieldfrequencies are generated at the transmitter and transmitted either overwire or radio to the receiver. These some waves are applied to thedeflection plates or coils of the pick-up tube at the transmitter andthe viewing tube at the receiver, so that the electron ray in the tubesmust of necessity be always in step during the scanning action. When acommon power supply at a given frequency of cycles, for example, isavailable, only the voltage wave at the line frequency is transmitted,and voltage waves at the field frequency are generated locally at thetwo stations and locked in step by the common power supply. Because ofthe impossibility of this system getting out of synchronization as toboth line and frame frequencies, a much higher interlace ratio ispossible. This allows a given detail picture to be transmitted over amuch narrower frequency band, thereby allowing more transmitters tooperate in a given frequency band. Likewise, the transmission of muchhigher definition pictures over the same frequency band can alsobeaccomplished. j

Another advantage of this system is that it will receive pictures oftransmitters operating at any desired line and frame frequencies. Withthe present systems it is necessary that all transmitters operate withexactly the same respective scanning frequencies if all receivers are tobe capable of receiving from any one of the transmitters.

My invention resides in the improved method and system of the characterhereinafter described and claimed.

For the purpose of illustrating my invention, an embodiment thereof isshown in the drawings,

wherein Figures 1 and 1a. are simplified, diagrammatic views of atelevision transmitting station and a television receiving station,respectively, constructed and operating in accordance with my invention;

Figs. 2 and 2a are views similar to Figs. 1 and la, respectively,showing a modification;

Figs. 3 and 3a are views similar to Figs. 2 and 2a, respectively,showing another modification;

Figs'. 4 and 4a are simplified views, in block diagram, of a completetelevision system operating in accordance with my invention;

Figs. 5 and 5a are views similar to Figs. 4 and 4a, showing a.modification;

Fig. 6 is a schematic view showing an interl laced pattern possible .toobtain with my improved method and system; and

Fig. 7 is a detail, elevational view of a part which may be used inconnection with my invention.

With reference to Fig. l, the-numeral l0 designates a scanning device intheform of a cathoderay pick-up tube of any suitable, conventionalconstruction, and comprising a photosensitive screen II and an electrongun for developing a ray l2 of electrons directed at and focused on theadjacent surface of th'egscreen.

In operation, a light image of the view or object l3 for transmission isprojected by a lens system l4 onto the photosensitive screen I l, andthe electron ray I2 is deflected horizontally at a suitableline-frequency by the plates I 5 and is simultaneously deflectedvertically at a suitable held frequency by .the plates l5 so that thescreen I I is scanned.

Since the specific form of construction of the pick-up tube l8 does nothave any important bearing on my invention, and since the principle andmanner of operation of cathode-ray pick-up tubes of the general typeshown are well known to those skilled in the art, no detail explanationis believed to be necessary. The picture or video signals developed bythe tube l0 may be taken from a collector electrode ll, fed to anamplifier tube l8, and thence transmitted over a line [3 to thereceiving station.

At the transmitting station are deflecting circuits common with respectto the pick-up tube In and the cathode-ray receiver tube 20 at thereceiving station. One of these circuits comprises a gas-discharge tube2| and an amplifier tube 22, and generates a saw-tooth voltage wave atthe desired field frequency. In operation, the condenser 23 is chargedat a uniform rate until the plate of tube 2i reaches a potential whereationization takes place to cause practically instantaneous discharge ofthis condenser. Resistances 24, 25 and 26 are used to regulate thefrequency of the saw-tooth voltage wave generated, and a cathode-biasresistor 2! is effective to regulate the amplitude of the voltage wave.The sawtooth voltage wave is applied to the amplifier tube 22 through acoupling condenser 28 and an input resistance 23. The reference numerals30 and 3! designate, respectively, a cathode-bias resistance and a plateresistance for the tube 22.

The amplified saw-tooth voltage wave at the field frequency is fedthrough a coupling condenser 32 to the transmission line 33. Thisvoltage wave is impressed across the deflecting plates I5 through groundand a connection 34 from the line 33. Also, this same voltage wave isimpressed across the corresponding deflecting plates Hit of the receivertube 20 through ground and a connection 34a from the transmission line33.

The construction and principle of operation of the scanning device orcathode-ray tube 20 at the receiving station are similar to that of thetransmitter tube In, the main difference residing in the fact that thescreen of the receiver or viewing tube 20 is a fluorescent screen I lawhich may be formed on the inside surface of the end wall of this tube.

The deflecting circuit for generating a sawtooth voltage wave at thedesired line frequency comprises a gas-discharge tube 35 and anamplifier tube 36. This circuit is the same as the frame-deflectioncircuit except that the capacity of the condenser 31 is smaller thanthat of the corresponding condenser 23 since the frequency of thevoltage wave generated is higher. The amplified saw-tooth voltage waveat the line frequency is fed through a coupling condenser 38 to thetransmission line 39, from whence it is impressed across the deflectingplates l5 through ground and a connection 40 from the line 39. Also,this same voltage wave is impressed across the corresponding deflectingplates Ilia of the receiver tube 20 through ground and a connection 40afrom the transmission line 33.

At the receiving station, a connection 4| from the transmission line l8applies the amplified picture or video signals to the control grid orelectrode 42 of the electron gun in the tube 28, whereby the intensityof the ray I20, of electrons is modulated or made to vary in accordancewith the lights and shadows at the respective elemental areas of theobject I3 being televised. Since the electron rays l2 and l2a arescanning the respective screens H and Na in exactly the same manner andin synchronism, a visible image of the .object l3 will be produced onthe fluorescent screen Ila.

At the transmitting station, a transformer 43 supplies the necessaryheater voltages, and also has high voltage windings for the rectifiertubes 44 and 45. The rectifier tube 44 supplies voltage to the amplifiertubes I8, 22 and 38 and the gasdischarge tubes 2| and 35. The rectifiertube 45 supplies the necessary voltages for the pick-up tube l8.

Condensers 46, 41, 48 and 49 are filter condensers, and 50 designates afilter choke. Resistances 5i, 52, 53 and 54 are bleeder resistances forrectifier tube 44, and resistances 55,55, 51, 58 and 59 form the bleederfor rectifier tube 45.

When the two deflecting circuits are properly locked in at therespective desired frequencies, a resistance 80 may be used to providecoupling between these circuits.

If it is desired to have and maintain the line frequency an evenmultiple of the field frequency, this may be accomplished by aconnection 8| and a coupling condenser 62 between the plate of theamplifier tube 36 operating at line frequency and the grid of thegas-discharge tube operating at the field frequency. Otherwise, thisconnection and condenser are omitted.

Resistances 63 and 54 are connected across the bleeder and allow forpositioning of the beam pattern on the photosensitive screen ll.Resistances 65 and 88 connect the position controls to the deflectionplates, as shown.

"Ihe condensers 61, 68, 69 and 10 are by-pass condensers.

The picture or video signals taken from the collector electrode I! arefed to the amplifier tube l8 through a coupling condenser H and an inputresistance 12. A resistance 13 supplies plate voltage to the tube l8,and a resistance 14 applies the proper voltage to the collector electrode I I.

When the distance between stations is great, or in cases where thesignals are transmitted at low level, amplifiers may be used locally atthe receiving station, and supplied respectively from the connections34a, 40a and 4| to amplify the two saw-tooth voltages and the incomingvideo signals to sufiicient levels.

At the transmitter, a screen of the secondaryemissive type may be usedin lieu of a photosensitive screen.

For the purpose of transmitting sound simultaneously with the picture, amicrophone I5 and amplifier I6 are provided at the transmitter, thesound signals being sent over an independent transmission line 11 to aloudspeaker I8 at the receiving station.

At the receiver, a transformer 78 supplies any necessary heatervoltages, and also high voltage to a rectifier tube 80. The propervoltages are supplied to the receiver tube 20 by the circuit shown,comprising filter condensers 8| and 82, bleeder resistances 83, 84, 85and 88, and a bypass condenser 81. The correct bias is applied to thecontrol or modulating electrode 42 by a resistance 88, and the videosignals are applied to this electrode through a coupling condenser 88.

In the system shown in Figs. 2 and 2a, the construction and manner ofoperation of the pick-up and viewing tubes are the same as in Figs. 1and 1a.

Also, with the following exceptions, the two systems are the same, andfor simplicity of comparison, corresponding parts and connections in thetwo systems have been designated by the same respective referencecharacters.

In Figs. 2 and 2a there is the common linedefiection circuit at thetransmitting station,

comprising the gas-discharge tube and the amplifier tube 36, forgenerating and amplifying the saw-tooth voltage wave at the line fre-,quency, this wave being impressed across the deflection plates I6 ofthe pick-up tube I0 and transmitted as before over the line 39 andconnection 40a and impressed across the deflection plates l6a of theviewing tube 20. However, the receiving station has'its ownfield-deflection circuit '''comprising a gas-discharge tube 2Ia and anamplifier tube 22a corresponding to and operating in the same manner asthe tubes 2| and 22, respectively, in Fig. 2.

That is, in Fig. 2, the tubes 2| and 22 operate as in Fig. 1 to generatea saw-tooth voltage wave at the field frequency which is impressedacross deflection plates I5 by the line 33 and connection 34, but thewave is not transmitted as in Fig. 1. At the receiver, the tubes Zia and22a operate to generate asaw-tooth voltage wave at the'field frequencywhich is impressed across deflection plates I5a by the line 33a.

For, the purpose of maintaining operation of the two field-deflectioncircuits at the respective stations at the same desired field frequency,a common and local source of 60-cycle alternating current is utilized.Asshown, the GO-cycle wave is applied to the grid of both gas-dischargetubes 2| and 2Ia.

In Fig. 2a there is, in addition, the rectifier tube 9| and associatedbleeder circuit for supplying voltage to the tubes 2la and 22a.

In the system shown in Figs. Band 3a, the construction and manner ofoperation of the pick-up and viewing tubes are the same as in Figs. 1,la, 2 and 2a.

Also, with the following exceptions, the system in Figs. 3 and 3a is thesame as that in Figs. 2 and 2a, and for simplicity of comparison,corresponding parts and connections in these two systems have beendesignated by the same reference characters. In Fig. 3, the saw-toothvoltage wave at the field frequency is generated by a half-waverectifier tube 92 and the associated filter arrangement comprising afilter condenser 93 and the resistance 94 connected across thiscondenser. With each positive halfcycle of the 60-cycle power"frequency, the condenser 93 is charged, and this charge leaks out byway of the resistance 94, thereby generating a 60-cycle saw-toothvoltage wave with a somewhat longer return trace than by the method inFigs. 1 and 2. In. the systems described, this is not objectionablebecause there is effective scanning during the return trace as well asduring the initial trace.

In Fig. 3a, the difference over Fig. 2a resides also in the deflectingcircuit for generating the saw-tooth voltage wave at the fieldfrequency. That is, as in Fig. 3, a half-wave rectifier 92a and a filtercondenser 93a and resistance 94a are used for this purpose.

In the transmitting station shown in Fig. 4, the output from thecathode-ray pick-up tube 95 is fed through a preamplifier 96 and a mainamplifier 91 to the transmitter 98. A generator 99 operates to developthe saw-tooth voltage wave.

at line frequency which is fed to the sound amplifier I00 andtransmitter III. whereby this wave is transmitted on the sound carrier.A generator I02 operates to develop asaw-tooth voltage wave at the fieldfrequency of 60 cycles, and is held at this frequency by a local(SO-cycle power supply I93.

From the foregoing it will be understood that in Fig. 4 the videosignals are transmitted on one carrier wave and the saw-tooth voltagewave for' deflecting the cathode-ray at line frequency is transmittedwith the sound signals on the sound carrier wave.

At the receiving station, shown in'Fig. 4a, the signals are amplified bya radio-frequency amplifier I04. A locally generated oscillation ismixed with the signals, as represented, and fed to the videointermediate-frequency amplifier I95 and the audiointermediate-frequency amplifier I06. The video signals are thendemodulated, amplified, and applied to the control grid of the viewingtube I01 to modulate the intensity of the electron rat.

The audio signals and the saw-tooth voltage v wave at line frequencybeing on a difierent carrier wave, do not interfere with the videosignals, and are separated and pass through their ownintermediate-freque'ncy amplifier I06 and are then demodulated. Thevoltage wave passes through a selective filter I08, and after beingamplified by an amplifier Ill9is impressedacross the deflection platesof the viewing tube III! to cause deflection of the electron ray at theline frequency.

The sound signals are amplified by an amplifier I I0 and fed to theloudspeaker I.

As in Figs. 2a and 3a,a local generator H3 operates to develop thesaw-tooth voltage wave at the field frequency, andis held at thisfrequency by the local 60-cycle power supply Ill common to bothstations, as in Figs. 2, 2a and Figs. 3, 3a.,

In Figs-5 and 5a no sound is transmitted, and

, both saw-tooth waves, at the line and field frequencies respectively,are transmitted on the sound carrier. That is, at the transmitter, theoutputs from both deflection circuits 99 and III! are fed to theaudioamplifier I00 and then pass to the transmitter Ill]v fortransmission on the sound carrier, as represented in Fig. 5. Theoperation, otherwise, is the same as in Fig. 4. At the receiver, nolocal field generator, such as the generator H3 in Fig. 4a, isnecessary, since the saw-tooth voltage wave at the field frequency isreceived from the transmitter, along with the saw-tooth voltage waveat.'the line frequency. These waves are demodulated, and separated bythe respective filters I08 and H6, and then amplified by the respectiveamplifiers I99 and H1 before being impressed across the deflectionplates.

In cases where a secondary-emissive screen is used at any of thetransmitting stations in lieu of a photosensitive screen, one method forthis purpose might be to employ an aluminum plate with letters,numerals, or any desired figure or view printed, with a carbon ink, onthe surface to be scanned. As the printed surface is scanned by theelectron ray, the voltage output is different when'the ray is directedat the aluminum surface than when it is directed at the carbon printing,whereby the picture signals are devel-.

oped in accordancewith the printing scanned by the electron'ray. It willbe understood that in this method different transmitter tubes arerequired for different messages, since the message transmitted from anyone tube is fixed by the particular carbon printing on the aluminumplate. However, if for example letters of the alphabet are printed onthe aluminum plate and arranged in some such manner as in Fig. 7, and atthe transmitter the electron ray is positioned so that it scans only oneletter or figure at a time and under control of the operator, only suchletter or figure will appear on the fluorescent screen of the viewingtube. In this way, messages can be sent, such as by spelling out codewords.

Code messages can also be transmitted by using a conventional type ofviewing tube at the receiving station and placing in front of thefluorescent screen a transparent card with the letters of the alphabetthereon and arranged in some such manner as in Fig. 7. At thetransmitter, no picture signals are developed, but only the two sawtoothvoltage waves at the respective line and field frequencies aretransmitted to the receiver, as in Figs. 1 and 1a and Figs. 5 and 50. Bycontrolling the relation of these waves at the transmitter, the scannedarea at the viewing tube can be positioned or selected to embrace anyone letter or numeral on the card.

By actually transmitting to the receiver the saw-tooth wave at the linefrequency, and either transmitting also the saw-tooth wave at the fieldfrequency or using local field generators at the two stationssynchronized by a common power supply of a given frequency, asexplained, it follows that the two electron rays will always scan therespective screens in the same manner and in synchronism. For thisreason, it is possible in my improved system and method to obtain theadvantage of using lin and field frequencies to give an interlace ratiogreater than 2 and even as high as 4.

For example, in the various systems used heretofore, in whichcathode-raytubes are employed at both stations, and line and fieldimpulses are transmitted for synchronization, some success has been hadin obtaining an interlace ratio of 2. But even at this lowest interlaceratio, the line and field impulses sometimes fail to hold the interlacedpattern at the viewing tube.

Assuming a line frequency of 6615 and a field frequency of 60 in myimproved system, as shown in Figs. 1 to 511', there will be 110.25 linesper field, and accordingly a frame frequency of 15 and an interlaceratio of 4. That is, at each station, and

with reference to Fig. 6, the electron ray during the first field of anyone frame will scan the effective screen area ABCD along the solid lines1 to 110.25. During the second field of this frame, the electron rayswill scan the dot lines 111 to 220.50. During the third field of thisframe, the electron rays will scan the dash lines 221 to 330.75. Duringthe fourth field of this frame, the electron rays will scan the dash-dotlines 331 to 441, and then be at the initial starting point for the nextframe of four fields.

From all the foregoing it will be seen that in my improved method andsystem, practically one hundred percent of the transmitted energy forthe video signals is used for the same, since the saw-tooth voltage waveor waves are transmitted on the sound carrier. Also, no synchronizingpulses are required, and when both voltage waves are transmitted nodeflection circuits are required at the receiver.. When there isavailable a common power supply at a given frequency such as 60 cycles,it is only necessary to transmit the saw-tooth wave at the linefrequency since the saw-tooth voltage wave at the line frequency can ,begenerated locally at the two stations in a number of difierent waysandmust of necessity be in step. Any difference in phase at the twostations can be readily corrected for by a suitable phase-shiftingarrangement.

In the specification and claims, the following meaning is intended forthe various terms used. A frame is a single complete picture. Framefrequency is the number of times per second the frame or entireeffective screen area is completely scanned in interlaced scanning.Field frequency is the number of times per second the frame or entireeffective screen area is fractionally scanned in interlaced scanning.The interlace ratio is the ratio of field frequency to frame frequency.For other definitions, reference may be made to the publicationElectronics, issue of August, 1937.

It will be understood that various embodiments of my invention otherthan those disclosed, are

possible without departing from the spirit of my invention or the scopeof the claims.

I claim as my invention:

1. In the art of simultaneous television and sound communication betwentwo stations wherein cathode-ray tubes are ultilized at the respecsoundsignals on another carrier wave, de-

veloping a continuous electrical wave at a line frequency for deflectingthe scanning my at said first-named station at said line frequency,transmitting said continuous electrical wave on the sound-carrier tosaid other station from said firstnamed station, demodulating at saidother station to obtain thereat said electrical wave, and utilizing theelectrical wave so obtained to deflect the scanning ray at said otherstation at said line frequency.

2. In the art of simultaneous television and sound communication betweentwo stations wherein from one of said stations to the other stationpicture signals are transmitted on a carrier wave and sound signals aresimultaneously transmitted on another carrier wave, the method ofoperation which comprises utilizing a ray-deflecting wave to controlscanning action at said firstnamed station, transmitting saidray-deflecting wave on the sound carrier to said other station from saidfirst-named station, demodulating at said other station to reproducethereat said rayrefiecting wave, and utilizing the ray-deflecting waveso obtained at said other station to control scanning action at saidother station.

3. In the art of simultaneous television and sound communication betweentwo stations wherein cathode-ray tubes are utilized at the respectivestations for scanning, the method of operation which comprisestransmiting from one of said stations to the other station picturesignals on a carrier wave and sound signals simultaneously on anothercarrier wave, utilizing a raydeflecting wave to cause deflection at agiven line frequency of the scanning ray at said first-named station,transmitting said ray-deflecting waveon the sound carrier to said otherstation from said first-named station, demodulating at said otherstation to reproduce thereat said ray-deflecting wave, and utilizing theray-deflecting wave so obtained at said other station to causedeflection at said line frequency of the scanning ray at said otherstation.

4. In a system for television communication between two stations,scanning devices in the greases 5 form or! cathode-ray tubes locatedrespectively at said stations, means common with respect to said tubesand .located at one of said stations and operating to develop acontinuous electrical wave at a given line frequency, means forreproducing said continuous electrical wave at the other station, meansassociated with the cathode-ray tube at said flrst-named station andhaving impressed thereon said electrical wave and operating to deflectthe scanning ray at said line frequency, means associated with thecathode-ray tube at said other station and having impressed thereon saidelectrical wave and operating to'deflect the scanning ray at said linefrequency, a deflecting.

circuit associated with the cathode-ray tube at said flrst-namedstatlonfor deflecting the scanning ray at a given field frequency, a deflectingcircuit associated with the cathode-ray tube at 5 said other station forreflecting the scanning ray at said field frequency, a source ofalternatingcurrent common with respect to and local to said stations,and connections between said common source of alternating-current andsaid deflecting circuits for maintaining operating action of each of thelatter at said field frequency.

' ALLEN B. DU MONT.

CERTIFICATE OF CQRRECTION. Patent no. 2,186, 6514.. January' 9, 19140.

ALLEN B. 1311- MONT.

It is hereby certified that error appeais in the printed specificationof theabeve numbered patent requiring"-correction as follows: Page 1;,sec-- ond eolumn, line 55, claim 2, and page 5 second column, line 6,claim 11., for the word "reflecting read -d.ef1ecting--; and that thesaid Letters Patent should be read ,with this correction'therein thatthe same may conform to: the record pf the'case in the Patent Office.

Signed and sealed this 18th day of June, A. D.'19L .O

I Henry Van Arsdale (Sea1) Acting Commissioner of Patents.

